Devices Employing Thermal and Mechanical Enhanced Layers and Methods of Forming Same

ABSTRACT

A method includes attaching a first-level device die to a dummy die, encapsulating the first-level device die in a first encapsulating material, forming through-vias over and electrically coupled to the first-level device die, attaching a second-level device die over the first-level device die, and encapsulating the through-vias and the second-level device die in a second encapsulating material. Redistribution lines are formed over and electrically coupled to the through-vias and the second-level device die. The dummy die, the first-level device die, the first encapsulating material, the second-level device die, and the second encapsulating material form parts of a composite wafer.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a divisional of U.S. application Ser. No.15/058,818, filed Mar. 2, 2016, and entitled “Devices Employing Thermaland Mechanical Enhanced Layers and Methods of Forming Same”, whichclaims the benefit of the following provisionally filed: U.S.Provisional Application No. 62/275,550, filed Jan. 6, 2016, and entitled“Devices Employing Thermal and Mechanical Enhanced Layers and Methods ofForming Same,” which application is hereby incorporated herein byreference.

BACKGROUND

Stacked dies are commonly used in Three-Dimensional (3D) integratedcircuits. Through the stacking of dies, the footprint (form factor) ofpackages is reduced. In addition, the metal line routing in the dies issignificantly simplified through the formation of stacked dies.

In some conventional applications, a plurality of dies is stacked toform a die stack, wherein the plurality of dies includeThrough-Substrate Vias (TSVs, sometimes known as through-silicon vias).The total count of the stacked dies may sometimes reach eight or more.When such a die stack is formed, a first die is first bonded onto apackage substrate through flip-chip bonding, wherein solderregions/balls are reflowed to join the first die to the packagesubstrate. A first underfill is dispensed into the gap between the firstdie and the package substrate. The first underfill is then cured. A testis then performed to ensure that the first die is connected to thepackage substrate properly, and that the first die and the packagesubstrate function as desired.

Next, a second die is bonded onto the first die through flip-chipbonding, wherein solder regions/balls are reflowed to join the seconddie to the first die. A second underfill is dispensed into the gapbetween the second die and the first die. The second underfill is thencured. A test is then performed to ensure that the second die isconnected to the first die and the package substrate correctly, and thefirst die, the second die, and the package substrate function asdesired. Next, a third die is bonded onto the second die through thesame process steps as for bonding the first die and the second die. Theprocesses are repeated until all the dies are bonded.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1 through 10 illustrate the cross-sectional views of intermediatestages in the formation of a multi-stacking fan-out package inaccordance with some embodiments.

FIGS. 11A, 11B, and 12 through 14 illustrate the cross-sectional viewsof intermediate stages in the formation of a multi-stacking fan-outpackage in accordance with some embodiments.

FIGS. 15 through 21 illustrate the cross-sectional views of intermediatestages in the formation of a multi-stacking fan-out package inaccordance with some embodiments.

FIG. 22 illustrates a top view of a multi-stacking fan-out package inaccordance with some embodiments.

FIG. 23 illustrates a process flow for forming a multi-stacking fan-outpackage in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,”“lower,” “overlying,” “upper” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

Integrated multi-stacking fan-out packages and the methods of formingthe same are provided in accordance with various exemplary embodiments.The intermediate stages of forming the multi-stacking fan-out packagesare illustrated. Some variations of some embodiments are discussed.Throughout various views and illustrative embodiments, like referencenumbers are used to designate like elements.

FIGS. 1 through 10 illustrate the cross-sectional views of intermediatestages in the formation of a multi-stacking fan-out package inaccordance with some embodiments. The steps shown in FIGS. 1 through 10are also illustrated schematically in the process flow 200 as shown inFIG. 23.

Referring to FIG. 1, wafer 24 is adhered to carrier 20. The respectivestep is illustrated as step 202 in the process flow shown in FIG. 23. Inaccordance with some embodiments of the present disclosure, carrier 20is a glass carrier. In accordance with other embodiments, carrier 20 isformed of other materials that are rigid. Adhesive film 22 may be usedfor attaching wafer 24 to carrier 20. Wafer 24 may have a round top-viewshape. Wafer 24 is referred to as a dummy wafer throughout thedescription since it may be a blank wafer with no active devices (suchas transistors and diodes) and passive devices (such as resistors,capacitors, and inductors) formed therein. Wafer 24 is formed of a rigidmaterial, which may have a Young's modulus equal to or greater than theYoung's modulus of silicon (about 165 GPa to about 179 GPa).Accordingly, dummy wafer 24 may have a Young's modulus equal to orgreater than about 165 GPa.

In addition, dummy wafer 24 may have a good thermal conductivity. Thethermal conductivity of dummy wafer 24 may be close to (for example,greater than 90 percent of) the thermal conductivity of thesemiconductor substrates (such as silicon substrates) in the overlyingdevice dies. For example, silicon has a thermal conductivity equal toabout 148 W/(m*K), and hence the thermal conductivity of dummy wafer 24may be greater than about 135 W/(m*K) or higher. With dummy wafer 24having a high thermal conductivity, the thermal dissipation in theresulting structure is improved.

In accordance with some embodiments of the present disclosure, dummywafer 24 is formed of a metal or a metal alloy, a semiconductormaterial, or a dielectric material. For example, when including metal,dummy wafer 24 may be formed of copper, aluminum, nickel, or the like,and hence is a metal film/plate in accordance with some embodiments.When formed of a semiconductor material, wafer 24 may be a siliconwafer, which may be the same type of wafer on which active devices areformed. When formed of a dielectric material, dummy wafer 24 may beformed of ceramic. In addition, the material of dummy wafer 24 may behomogenous. For example, the entire dummy wafer 24 may be formed of thesame material, which includes same elements, and the atomic percentagesof the elements may be uniform throughout dummy wafer 24. In accordancewith some exemplary embodiments, dummy wafer 24 is formed of silicon,with a p-type or an n-type impurity doped in dummy wafer 24. Inaccordance with alternative embodiments, no p-type impurity and n-typeimpurity are doped in dummy wafer 24.

Referring to FIG. 2, dummy wafer 24 may be thinned, for example, in agrinding process. The respective step is illustrated as step 204 in theprocess flow shown in FIG. 23. The resulting thickness T1 of dummy wafer24 is great enough so that dummy wafer 24 may provide adequatemechanical support to the overlying structures, which are built insubsequent steps.

Referring to FIG. 3, device dies 26 (including 26A and 26B) are adheredto dummy wafer 24 through Die Attach Films (DAFs) 32. The respectivestep is illustrated as step 206 in the process flow shown in FIG. 23.Throughout the description, device dies 26 are referred to as afirst-level device dies. The edges of DAFs 32 are co-terminus with(aligned to) the respective edges of device dies 26. In accordance withsome embodiments of the present disclosure, device dies 26 are memorydies, which may be Dynamic Random Access Memory (DRAM) dies,Negative-AND (NAND) dies, Static Random Access Memory (SRAM) dies,Double-Data-Rate (DDR) dies, or the like. Device dies 26 may also belogic device dies or integrated passive device dies (with no activedevices therein). Each of device dies 26 may be a single memory die or amemory die stack. Also, device dies 26 also include semiconductorsubstrates 25, wherein active devices (not shown) such as transistorsand/or diodes are formed at the top surfaces of semiconductorsubstrates. The back surfaces of device dies 26, which may also be theback surfaces of semiconductor substrates 25, are in contact with DAFs32.

Device dies 26 have thicknesses T2, and semiconductor substrates 25 indevice dies 26 have thickness T3. In accordance with some embodiments,thickness T1 of dummy wafer 24 is equal to or greater than thickness T3of semiconductor substrates 25. Thickness T1 may also be equal to orgreater than thickness T2 of device dies 26. Dummy wafer 24 has thefunction of providing mechanical support to the overlying structure.Accordingly, the material of dummy wafer 24 is selected to be thick andrigid enough. For example, thickness T1 of dummy wafer 24 is desirablyto be greater than thickness T3 or T2 to provide enough mechanicalsupport.

In accordance with some embodiments, device dies 26 include electricalconnectors 28, which may be metal pillars or metal pads. Electricalconnectors 28 are electrically coupled to the integrated circuits (notshown) inside device dies 26. Electrical connectors 28 may be copperpillars, and may also include other conductive/metallic materials suchas aluminum, nickel, or the like. In accordance with some exemplaryembodiments of the present disclosure, electrical connectors 28 are indielectric layers 30, with the top surfaces of dielectric layers 30being higher than or coplanar with the top surfaces of electricalconnectors 28. Dielectric layers 30 further extend into the gaps betweenelectrical connectors 28. Dielectric layers 30 may be formed of apolymer such as polybenzoxazole (PBO) or polyimide in accordance withsome exemplary embodiments.

Electrical connectors 28 may be offset from the centers of respectivedevice dies 26. For example, the electrical connectors 28 of theleft-side device die 26 (marked as 26A) are disposed on the left side ofdevice die 26A, while no electrical connector 28 is formed either closeto the center or on the right side of device die 26A. The electricalconnectors 28 of the right-side device die 26 (marked as 26B), on theother hand, are disposed on the right side of device die 26B, while noelectrical connector 28 is formed either close to the center or on theleft side of device die 26B.

FIG. 4 illustrates the encapsulation of encapsulating material 33, whichmay be a molding compound, a molding underfill, a resin, or the like inaccordance with some embodiments. The respective step is illustrated asstep 208 in the process flow shown in FIG. 23. Encapsulating material 33is dispensed as a fluid and then being compressed and cured, forexample, in a thermal curing process. Encapsulating material 33 fillsthe gaps between device die 26. After the encapsulating process, the topsurface of encapsulating material 33 is higher than the top ends ofelectrical connectors 28. Next, a planarization step such as amechanical grinding, a Chemical Mechanical Polish (CMP) and/or acombination of both is performed to planarize encapsulating material 33and electrical connectors 28.

In accordance with some embodiments, as shown in FIG. 4, dielectriclayer 35 is formed over device dies 26 and encapsulating material 33.Dielectric layer 35 may be formed of a polymer such as PBO, polyimide,BCB, or the like. Dielectric layer 35 is then patterned to expose theunderlying electrical connectors 28. Next, seed layer 36 is formed. Seedlayer 36 may include a titanium layer and a copper layer over thetitanium layer. Seed layer 36 extends into the openings in dielectriclayer 35 to contact, and electrically coupling to, electrical connectors28.

FIGS. 4 through 6 further illustrate the formation of through-vias 34.The respective step is illustrated as step 210 in the process flow shownin FIG. 23. Referring to FIG. 4, mask layer 38 is formed over seed layer36, and is then patterned to form openings 40, through which someportions of seed layer 36 are exposed.

As shown in FIG. 5, through-vias 34 are formed in openings 40 throughplating. Mask layer 38 is then removed, resulting in the structure inFIG. 6. In accordance with some embodiments of the present disclosure,after the removal of mask layer 38, the portions of seed layer 36 notdirectly underlying through-vias 34 are removed in an etching process.The remaining portions of seed layer 36 thus become the bottom portionsof through-vias 34. Throughout the description, through-vias 34 refer tothe portions of the plated material and seed layer 36 protruding higherthan the top surface of dielectric layer 35. The portions of the platedconductive material and seed layer extending into dielectric layer 35are referred to as vias, which connect the overlying through-vias 34 tothe underlying electrical connectors 28.

Next, referring to FIG. 7, device dies 42 are attached to dielectriclayer 35 through DAFs 44, wherein the back surface of device dies 42 areattached to the front surfaces of device dies 26. Throughout thedescription, device dies 42 are referred to as second-level device dies.The respective step is illustrated as step 212 in the process flow shownin FIG. 23. Device dies 42 may include electrical connectors 46 embeddedin the respective dielectric layers 48, wherein dielectric layers 48 maybe formed of a polymer such as PBO, polyimide, BCB, or the like.

Device dies 42 may be DRAM dies, NAND dies, SRAM dies, DDR dies, or thelike. Device dies 42 may also be logic device dies or integrated passivedevice dies (with no active devices therein). Furthermore, device dies42 and device dies 26 may be the same type of dies (for example, bothare DRAM dies), or may be different types of dies. As shown in FIG. 7,the first-level device dies 26 may have electrical connectors 28 offsetfrom the centers of the respective dies, and hence second-level devicedies 42 may overlap device dies 26. Using the device die 42 on the leftside of FIG. 7 as an example, device die 42 overlaps the right portionof the respective underlying device die 26A and the left portion of therespective underlying device die 26B.

In a subsequent step, encapsulating material 50, which may be a moldingcompound, a molding underfill, a resin, or the like is encapsulated onthrough-vias 34 and device dies 42. The respective step is illustratedas step 214 in the process flow shown in FIG. 23. Next, a planarizationstep such as a mechanical grinding, CMP or a combination of both isperformed to planarize encapsulating material 50 and device dies 42, sothat electrical connectors 46 and through-vias 34 are exposed. In theresulting structure, through-vias 34 penetrate through encapsulatingmaterial 50.

FIG. 8 illustrates the formation of dielectric layer 52 and through-vias54. The respective step is illustrated as step 216 in the process flowshown in FIG. 23. In the respective formation process, dielectric layer52 is first formed, followed by the patterning of dielectric layer 52 toexpose the underlying electrical connectors 46 and through-vias 34.Dielectric layer 52 may be formed of PBO, polyimide, BCB, or the like.Next, through-vias 54 are formed. The formation steps of through-vias 54may be similar to the process steps shown in FIGS. 4, 5, and 6, andhence are not repeated herein.

As also shown in FIG. 8, device dies 56 are attached to dielectric layer52 through DAFs 58. Device dies 56 are also referred to as third-leveldevice dies throughout the description. The respective step is alsoillustrated as step 216 in the process flow shown in FIG. 23. Inaccordance with some embodiments of the present disclosure, device dies56 are System on Chip (SoC) dies. Device dies 56 may be logic dies,which may be Central Processing Unit (CPU) dies, Micro Control Unit(MCU) dies, Input-Output (IO) dies, BaseBand (BB) dies, or Applicationprocessor (AP) dies. Although not shown, device dies 56 includesemiconductor substrates, wherein active devices such as transistorsand/or diodes are formed at the top surfaces of the semiconductorsubstrates. Furthermore, metal lines and vias (not shown) are formed ininterconnect structures (not shown), which are over the respectivesemiconductor substrates, to interconnect the integrated circuit devicesin device dies 56. Device dies 56 further include electrical connectors60 embedded in the respective dielectric layers 62, wherein thematerials of electrical connectors 60 and dielectric layers 62 may besimilar to the materials of electrical connectors 28 and dielectriclayers 30, respectively.

Referring to FIG. 9, encapsulating material 59, which may be a moldingcompound, a molding underfill, a resin, or the like is encapsulated onthrough-vias 54 and device dies 56. The respective step is illustratedas step 218 in the process flow shown in FIG. 23. Next, a planarizationstep is performed to planarize encapsulating material 59 and device dies56, so that electrical connectors 60 and through-vias 54 are exposed.

Further referring to FIG. 9, one or more dielectric layer(s) 64 and therespective Redistribution Lines (RDLs) 66 are formed over encapsulatingmaterial 59, through-vias 54, and device dies 56. In accordance withsome embodiments of the present disclosure, dielectric layers 64 areformed of a polymer(s) such as PBO, polyimide, BCB, or the like.

RDLs 66 are formed in dielectric layers 64. The respective step isillustrated as step 220 in the process flow shown in FIG. 23. RDLs 66are connected to the respective underlying electrical connectors 60and/or through-vias 54. RDLs 66 may also connect some electricalconnectors 60 to through-vias 54. RDLs 66 may include metal traces(metal lines) and vias underlying and connected to the metal traces. Inaccordance with some embodiments of the present disclosure, RDLs 66 areformed through plating processes, wherein each of RDLs 66 includes aseed layer (not shown) and a plated metallic material over the seedlayer. The seed layer and the plated metallic material may be formed ofthe same material or different materials.

FIG. 9 further illustrates the formation of electrical connectors 68 inaccordance with some exemplary embodiments of the present disclosure.The respective step is also illustrated as step 220 in the process flowshown in FIG. 23. Electrical connectors 68 are electrically coupled toRDLs 66, electrical connectors 60, and/or through-vias 54. The formationof electrical connectors 68 may include placing solder balls over RDLs66 and then reflowing the solder balls. In accordance with alternativeembodiments of the present disclosure, the formation of electricalconnectors 68 includes performing a plating step to form solder regionsover RDLs 66, and then reflowing the solder regions. Electricalconnectors 68 may also include metal pillars, or metal pillars andsolder caps, which may also be formed through plating. Throughout thedescription, the portions of the structure over adhesive film 22 are incombination referred to as composite wafer 70.

In subsequent steps, carrier 20 is de-bonded from composite wafer 70.The respective step is illustrated as step 222 in the process flow shownin FIG. 23. Composite wafer 70 is sawed apart into a plurality ofpackages 72, wherein one of packages 72 is shown in FIG. 10. Therespective step is illustrated as step 224 in the process flow shown inFIG. 23.

Referring to FIG. 10, package 72 is a multi-level (multi-stacking)package including two levels, three levels, or more levels of devicedies. Furthermore, there may be multiple levels of through-vias andencapsulating materials. Dummy wafer 24 and DAF 22 are sawed into dummydies 24′ and DAFs 22′ respectively.

FIGS. 11 through 14 illustrate cross-sectional views of intermediatestages in the formation of a multi-level package in accordance with someembodiments of the present disclosure. Unless specified otherwise, thematerials and the formation methods of the components in theseembodiments are essentially the same as the like components, which aredenoted by like reference numerals in the embodiments shown in FIGS. 1through 10. The details regarding the formation process and thematerials of the components shown in FIGS. 11A through 14 (and FIGS. 15through 21) may thus be found in the discussion of the embodiments shownin FIGS. 1 through 10.

The embodiments shown in FIGS. 11A through 14 are similar to theembodiments in FIGS. 1 through 10, except no carrier is used, and thebonding of the first-level device dies 26 to dummy wafer 24 may bethrough direct bonding rather than through DAFs. FIG. 11A illustratesthe bonding of device dies 26 onto dummy wafer 24. Dummy wafer 24 is athick wafer that has not been thinned yet. Accordingly, carrier may notbe used in accordance with some embodiments, although a carrier may alsobe used. Device dies 26 include semiconductor substrates 25, which maybe silicon substrates. In accordance with some embodiments of thepresent disclosure, the back surfaces of substrates 25 are in directbond with dummy wafer 24, which may be a silicon wafer. The bonding mayform Si-Si bonds. FIG. 11B illustrates the bonding in accordance withalternative embodiments, wherein silicon oxide layer 23 is formed as thetop surface portion of dummy wafer 24, for example, through thermaloxidation of dummy wafer 24. Semiconductor substrates 25 of device dies26 are bonded to dielectric layer 23 (which may be a silicon oxidelayer) through fusion bonding. In accordance with some embodiments,Si—O—Si bonds are formed to bond substrates 25 to dielectric layer 23.

The subsequent process steps in accordance with these embodiments aresimilar to what are shown in FIG. 4 through 9, and the processes andmaterials may be found referring to the embodiments of FIGS. 4 through9. The resulting structure is shown in FIG. 12. In FIG. 12, dielectriclayer 23 is illustrated using dashed lines to indicate it may or may notexist. Composite wafer 70, which includes dummy wafer 24 and theoverlying device dies, encapsulating materials, through-vias, etc., isthus formed.

FIG. 13 illustrates the thinning of dummy wafer 24, for example, throughmechanical grinding. In a subsequent step, composite wafer 70 is sawedinto packages 72, wherein one of packages 72 is shown in FIG. 14.

FIGS. 15 through 21 illustrate the cross-sectional views of intermediatestages in the formation of a multi-level package in accordance with someembodiments of the present disclosure. These embodiments are similar tothe embodiments shown in FIGS. 1 through 10, except that instead ofusing a dummy wafer, discrete dummy dies are used.

Referring to FIG. 15, dummy dies 24′ are attached to carrier 20 throughDAFs 22′. Dummy dies 24′ and the respective underlying adhesive films22′ are pre-cut as small pieces. The material of dummy dies 24′ may beselected from the same candidate materials of dummy wafer 24 (FIG. 1).In accordance with some embodiments, dummy dies 24′ are obtained bysawing dummy wafer 24 that may be used in the embodiments shown in FIGS.1 through 10. Carrier 20 may have a round top-view shape, and dummy dies24′ may be arranged as an array. The subsequent process steps, which areshown in FIGS. 16, 17, 18, and 19 in accordance with some embodimentsare similar to what are shown in FIG. 4 through 9, and the respectiveprocesses and materials may be found referring to the embodiments shownin FIGS. 4 through 9. The resulting structure is shown in FIG. 20. InFIG. 20, the portions of the structure over carrier 20 are incombination referred to as composite wafer 70 hereinafter.

Next, carrier 20 is de-bonded from composite wafer 70, followed byperforming a die-saw on composite wafer 70. In the resulting package 72,as shown in FIG. 21, encapsulating material 33 extends to a level lowerthan the bottom surface of dummy die 24′, and encircles dummy die 24′.Encapsulating material 33 may also encircle DAF 22′. In accordance withsome embodiments of the present disclosure, the bottom surface ofencapsulating material 33 is coplanar with the bottom surface of DAF22′. In accordance with some embodiments, after the de-bonding ofcarrier 20 and before the die-saw, a backside grinding is performed toremove adhesives 22′, so that dummy dies 24′ may be exposed. In theresulting package 72, the bottom surface of encapsulating material 33 iscoplanar with the bottom surface of dummy die 24′.

FIG. 22 illustrates a top view of package 72 in accordance with someembodiments. Encapsulating materials 33 encircles first-level devicedies 26, and extends into the gap between device dies 26A and 26B.Encapsulating material 50 encircles second-level device die (or devicedies) 42 and through-vias 34. Encapsulating material 59 encirclesthird-level device die (or device dies) 56 and through-vias 54. When theembodiments shown in FIGS. 1 through 14 are adopted, in the resultingpackage 72, dummy die 24′ extends all the way to the edges of package72. The edges of dummy die 24′ thus co-terminate with the edges ofencapsulating materials 33, 50, and 59. Alternatively, when theembodiments shown in FIGS. 15 through 21 are adopted, in the resultingpackage 72, dummy die 24′ extends beyond the edges of device dies 26,42, and 56, and the edges of dummy die 24′ do not reach the edges ofpackage 72.

The embodiments of the present disclosure have some advantageousfeatures. Multi-stacking packages may become very thin, even thoughthere may be multiple-levels of device dies, in order to suit to therequirement of demanding applications such as mobile applications. Thethin multi-stacking packages thus suffer from warpage. The warpage isfurther worsened when elongated device dies (refer to the top-viewshapes of device dies 26 in FIG. 22) are used. Accordingly, a rigiddummy die is added into the multi-stacking package to provide mechanicalsupport, so that warpage is reduced. The dummy die is also formed of amaterial having a good thermal conductivity, so that the dummy die canreadily conduct heat out of the package, and the thermal dissipation ofthe multi-stacking package is improved.

In accordance with some embodiments of the present disclosure, a methodincludes attaching a first-level device die to a dummy die,encapsulating the first-level device die in a first encapsulatingmaterial, forming through-vias over and electrically coupled to thefirst-level device die, attaching a second-level device die over thefirst-level device die, and encapsulating the through-vias and thesecond-level device die in a second encapsulating material.Redistribution lines are formed over and electrically coupled to thethrough-vias and the second-level device die. The dummy die, thefirst-level device die, the first encapsulating material, thesecond-level device die, and the second encapsulating material formparts of a composite wafer.

In accordance with some embodiments of the present disclosure, a methodincludes attaching a dummy wafer over a carrier. The dummy wafer is freefrom integrated circuit devices. The method further includes thinningthe dummy wafer, attaching first-level device dies to the thinned dummywafer, stacking second-level device dies over the first-level devicedies, forming through-vias electrically coupled to the first-leveldevice dies, and forming redistribution lines over and electricallycoupled to the through-vias and the second-level device die. A die-sawis performed to separate the dummy wafer, the first-level device dies,and the second-level device dies into a plurality of packages. Each ofthe plurality of packages includes a dummy die in the dummy wafer, oneof the first-level device dies, and one of the second-level device dies.

In accordance with some embodiments of the present disclosure, a packageincludes a dummy die, a first-level device die over and attached to thedummy die, a first encapsulating material encapsulating the first-leveldevice die, a second-level device die over the first-level device die,and a plurality of through-vias overlapping and electrically connectedto the first-level device die. The plurality of through-vias is at asame level as the second-level device die. A second encapsulatingmaterial encapsulates the second-level device die and the plurality ofthrough-vias therein. Redistribution lines are over and electricallycoupled to the plurality of through-vias and the second-level devicedie.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A package comprising: a dummy die; a first-leveldevice die over and attached to the dummy die; a first encapsulatingmaterial encapsulating the first-level device die; a second-level devicedie over the first-level device die; a plurality of through-viasoverlapping and electrically connecting to the first-level device die,wherein the plurality of through-vias are at a same level as thesecond-level device die; a second encapsulating material encapsulatingthe second-level device die and the plurality of through-vias therein;and redistribution lines over and electrically coupling to the pluralityof through-vias and the second-level device die.
 2. The package of claim1, wherein the dummy die comprises a silicon plate.
 3. The package ofclaim 1, wherein the dummy die comprises a metal plate.
 4. The packageof claim 1, wherein the first-level device die is directly bonded to thedummy die, with no adhesive film therebetween.
 5. The package of claim 1further comprising an adhesive film bonding the first-level device dieto the dummy die.
 6. The package of claim 1, wherein the firstencapsulating material comprises a portion encircling the dummy die, andthe portion of the first encapsulating material is level with the dummydie.
 7. The package of claim 1 further comprising: a dielectric layeroverlapping the first-level device die and the first encapsulatingmaterial, wherein the dielectric layer separates the first encapsulatingmaterial from the second encapsulating material.
 8. A packagecomprising: a rigid dummy die; a first die-attach film and a seconddie-attach film over the rigid dummy die; a first device die and asecond device die attached to the rigid dummy die through the firstdie-attach film and the second die-attach film, respectively; a firstencapsulating material encapsulating the first device die, the seconddevice die, and the rigid dummy die therein; a dielectric layeroverlapping the first device die, the second device die, and the firstencapsulating material; a third device die over the dielectric layer,wherein the third device die comprises a first portion overlapping aportion of the first device die, and a second portion overlapping aportion of the second device die; and a second encapsulating materialencapsulating the third device die therein.
 9. The package of claim 8further comprising: a through-via penetrating through the dielectriclayer and the second encapsulating material, wherein the through-via hasa bottom surface contacting a top surface of an electrical connector inthe first device die.
 10. The package of claim 9, wherein a top surfaceof the through-via is substantially coplanar with a top surface of thesecond encapsulating material.
 11. The package of claim 8 furthercomprising a second dielectric layer overlapping and contacting a topsurface of the second encapsulating material and a top surface of thethird device die.
 12. The package of claim 8, wherein the firstencapsulating material further encapsulates the first die-attach filmand the second die-attach film therein.
 13. The package of claim 8,wherein the rigid dummy die comprises silicon, and the rigid dummy diehas no active devices therein.
 14. The package of claim 8 furthercomprising: a fourth device die overlapping a first portion of the thirddevice die; a third encapsulating material encapsulating the fourthdevice die therein; and an additional through-via overlapping a secondportion of the third device die, and the additional through-viapenetrates through the third encapsulating material.
 15. The package ofclaim 8, wherein respective edges of the first encapsulating material,the second encapsulating material, and the rigid dummy die are flushwith each other.
 16. A package comprising: a blank die free from activedevices, passive devices, and conductive lines therein; a first devicedie over and attached to the blank die through an adhesive film, whereinthe first device die comprises a conductive pillar; a firstencapsulating material encapsulating the first device die therein; adielectric layer comprising a bottom surface contacting a top surface ofthe first device die and a top surface of the first encapsulatingmaterial; a second encapsulating material over the dielectric layer; anda through-via penetrating through the second encapsulating material andthe dielectric layer to contact the conductive pillar of the firstdevice die.
 17. The package of claim 16, wherein the blank die extendsbeyond edges of the first device die.
 18. The package of claim 17,wherein edges of the blank die are flush with respective edges of thefirst encapsulating material and the second encapsulating material. 19.The package of claim 16, wherein the adhesive film is encapsulated inthe second encapsulating material.
 20. The package of claim 16 furthercomprising: a second device die encapsulated in the second encapsulatingmaterial, wherein the second device die overlaps a first portion of thefirst device die, and the second encapsulating material and thethrough-via overlap a second portion of the first device die.